1. Field of the Invention
The present invention relates to the field of high temperature superconductor (HTS) coated conductors with filamentary current sharing.
2. Description of the Related Art
The YBCO is a type II high temperature superconductor capable of large current densities (−106 A/cm2 at 77 K, self-field), and maintains these high current densities (−105 A/cm2 at 77 K, 2 T) in applied magnetic fields. Industry is working to scale up the HTS coated conductor to long production lengths. High current YBCO HTS tape is becoming available for a variety of applications such as power transmission cables, fault current limiters, high-field magnets, transformers, motors, and generators. A number of these applications such as transformers, motors, and generators require a superconductor with minimal AC loss in order to keep refrigeration requirements, hence weight and volume, to a minimum.
Currently, industry is focusing on development of the DC conductor, in which the YBCO thin film is deposited on a metal substrate separated by buffer layers. Multiple processes (pulsed laser deposition, metal organic chemical vapor deposition, sol-gel methods, metal organic deposition, etc.) are used to deposit the YBCO film on a metal substrate textured by either the RABiTs or IBAD technique. The thin YBCO film is approximately ˜0.3 μm to a few μm's thick on a metal substrate which may be approximately 4 mm to 1 cm wide, although the metal substrate can be significantly wider for manufacturing. In this superconducting thin film, current flows in a surface layer, and the thickness of this layer depends on the critical current. For a filament, this current sheath reaches the center portion of the superconductor filament when the transport current is large enough; this is the fully penetrated case.
In the fully penetrated case, the hysteresis loss in the superconductor is proportional to the width of the filament perpendicular to both the applied field and the current flow. In applications such as superconducting generators and transformers, the conductor can be exposed to alternating magnetic fields (up to a few Tesla) at high frequencies (up to a couple thousand Hz).
Since YBCO requires strict biaxial alignment, the most suitable form for creating an HTS conductor with YBCO is as an HTS coated conductor. The preferential characteristics of the YBCO coated conductor, especially its in-field performance, indicate potential for using this conductor in power generation applications such as superconducting generators, motors, and transformers.
There are many AC losses that can be introduced in the HTS conductor when placed in the aforementioned applications. These include hysteretic losses in the superconducting layer, coupling losses in a filamented superconducting layer, eddy current losses in both the substrate and stabilizing layer, ferromagnetic losses of the substrate, and transport current losses. In developing a more AC-tolerant HTS conductor, subdividing a YBCO tape into filaments can significantly decrease the hysteresis losses in AC applications. Hysteresis losses in the conductor are realized as waste heat; minimizing these losses lower the refrigeration requirement. The resulting multifilamentary structure is a tape with parallel thin strips of YBCO material separated by non-superconducting, resistive barriers. This subdivision of the HTS tape into a multifilamentary structure serves to reduce the high aspect ratio of the thin film tapes. This is necessary since the hysteresis losses of a superconducting tape are directly proportional to the width of the conductor when fully penetrated by a magnetic field. The loss per volume per cycle can be given by the following equation in SI units:
      Q    V    ≈            1      10        ⁢          dj      c        ⁢          H      0      
where d is the filament width and H0 is the field amplitude which is large compared to the full penetration field Hp. Since filamentation of the YBCO coated conductor introduces coupling losses in AC fields, it is well known that the conductor will require twisting along its length. The twists in a flat HTS tape will be inherently volume inefficient.
An important consideration for lowering the losses in the YBCO coated conductor is that the filaments must be twisted or transposed. Bending the HTS tape itself, though possible, does have limitations due to the bend strain tolerance of the superconductor. For bending geometry, as opposed to twisting geometry, strain will be given by
  e  ≈            t      YBCO              t      cab      
where tYBCO is the thickness of the YBCO in a neutral axis geometry or the distance the YBCO layer is located from center. This gives, for a 0.5% strain limitation and a t=25 μm a limitation on the bend radius to be no less than 1 cm with consideration for margin of error.
Another important consideration for lowering the hysteresis losses in the YBCO coated conductor is that the filaments must be twisted or transposed. Bending the HTS tape itself, though possible, does have limitations due to the bend strain tolerance of the superconductor. Additionally, the twists in a flat tape will be volume inefficient in application.
Therefore, a need exists for HTS tape configuration and method to create such a configuration to transpose the filaments without physically bending the superconducting material itself. This type of configuration will reduce AC losses, allow for compact design of devices that utilize AC current and reduce associated refrigeration costs.